494 RADIATION BIOLOGY 



vation, were produced with a frequency which, at low doses and doses of 

 low intensity, followed approximately the first power of the dose. At 

 doses of higher intensity, as the dose was raised, their frequency followed 

 a power of the dose which approached more and more nearly 2. It was 

 reasonable to conclude from this result that, in the case of a part of these 

 small deficiencies (a large proportion of them at the lower doses), the two 

 breaks involved were produced in the course of a single electron track, 

 these cases being simply proportional in frequency to the dose, and that 

 in the rest, as in gross structural changes in general, the two breaks had 

 been produced by the hits of independent electron tracks, these cases 

 being proportional in frequency to the square of the dose. 



The deleted pieces dealt with by Rick, being readily visible in mitotic 

 stages, were, of course, much larger than those studied in the preceding 

 Drosophila experiments. However, the actual distance between the 

 breaks at the time when they were produced, measured not along the 

 chromosome thread but directly, may often have been no greater than for 

 the Drosophila cases since the chromosomes were doubtless sufficiently 

 coiled to bring points on neighboring rungs much closer physically than 

 they were along the chromosome thread. It would probably be this 

 physical distance between two given points, rather than their distance 

 along the chromosome thread, which would determine whether ions of 

 the same cluster were in ''striking range" of both points. 



As for evidence from plant material concerning still smaller deficien- 

 cies, it should be recalled that, according to Stadler's interpretation, many 

 of the point mutations obtained by him from X-irradiated cereals are 

 really small deletions and that some of these are small enough to be able 

 to pass down through the male gametophyte generation. Since he found 

 the frequency of the induced point mutations as a group to be simply 

 proportional to dose, it would follow that, if they are composed in any 

 considerable measure of small deletions, these deletions also follow the 

 dose in a linear manner. Thus their two breaks would be referable to 

 the same electron track. 



In regard to the deletions formed within euchromatic regions of Dro- 

 sophila chromosomes the evidence still seems conflicting. Earlier work 

 done by Makki under the direction of Muller (1940) had indicated a 

 linear frequency-dose relation for lethal point mutations of a group which 

 were presumed to be mainly small deletions. These were lethals which 

 showed no disturbance of crossing over and which had arisen in connec- 

 tion with phenotypic changes referable to genes ordinarily giving visible 

 viable mutations. It was inferred that, in these cases, two neighboring 

 breaks had been produced by one hit, or, as should now be said (less 

 committally), in the course of one electron track. 



Demerec and Fano (1941), in salivary-chromosome observations of a 

 considerable number of deletions, all of which included the locus of Notch 



